Image display systems based on spatial light modulators (SLMs) are increasingly being used as an alternative to image display systems based on cathode ray tubes. As used for image display applications, SLMs are arrays of pixel-generating elements that emit or reflect light to an image plane. The pixel-generating elements are often themselves referred to as "pixels", as distinguished from pixels of the image. This terminology is clear from context, so long as it is understood that more than one pixel of the SLM array can be used to generate a pixel of the image.
Digital micro-mirror devices (DMDs) are one type of SLM. A DMD has an array of hundreds or thousands of tiny tilting mirrors. To permit the mirrors to tilt, each is attached to one or more hinges mounted on support posts, and spaced by means of an air gap over underlying control circuitry. The control circuitry provides electrostatic forces, which cause each mirror to selectively tilt. Each mirror element provides the intensity for one pixel of the image.
The mirror elements of the DMD are individually addressable, such that the image is defined by which pixels are on or off at a given time. For addressing mirror elements of the DMD, each mirror element is in communication with a memory cell that stores a bit of data that determines the on or off state of the address signal. The addressing is binary in the sense that each mirror element is addressed with a high or low signal that indicates whether or not the m mirror element is to reflect light to the image plane. The DMD is "loaded" by storing input data in the memory cells, via a data loading circuit peripheral to the DMD's array of mirror elements.
Pixel data is delivered to the memory cells of the DMD in a special "bit-plane" format. This format arranges the data for each frame by the bit-weights of all pixels rather than pixel-by-pixel. This format permits greyscale images to be generated by addressing each mirror element with successive address signals during a frame period, each address signal representing a different bit weight of that mirror element's n-bit pixel value. For example, for 255 levels of non-black intensity, each mirror element could be addressed seven times during a frame. The more significant the bit-weight of the bit being used for addressing, the longer the mirror element would remain on. For the brightest intensity, the mirror element would be on each time it is addressed. This is essentially pulse width modulation, with many variations possible. Moving images can be generated by re-addressing the DMD with data for successive frames.
For color images, one approach is to use three DMDs, one for each primary color (R,G, B). The light from corresponding pixels of each DMD is converged so that the viewer perceives the desired color. Another approach is to use a single DMD and a color wheel having sections of primary colors. Data for different colors is sequenced and synchronized to the color wheel so that the eye integrates sequential images into a continuous color image. A third approach uses two DMDs, with one switching between two colors and the other displayed a third color.
As with all display systems, the quality of the images from a DMD-based display system is improved by eliminating artifacts. Potential artifacts include temporal contouring, which appears as flashing or banding when the observer blinks, moves his eyes, or waves his hands in front of his eyes. Another artifact is motion contouring, which appears as false contours that appear when the eye is tracking a moving object. The false contour may be a ghost image at sharp edges or an artificial contour in smoothly varying regions. Still another type of artifact is unique to DMD display systems that use a method of data loading known as memory-multiplexing.